This invention concerns the synthesis of extremely pure salt materials for use in glass products such as optical fibers, solid state lasers, and dielectric materials, wherein contaminants measured in just a few parts or less, per billion of the desired salt are often intolerable. In particular the invention concerns the purification of the chloro, bromo and iodo salts of zirconium, hafnium and aluminum, hereinafter termed "precursor salts", which can be reacted with fluorinating agents to give the ultra pure fluorides, hereinafter termed "product salts", in commercially desired quantities necessary for the preparation of high quality fluoride glasses, such as "ZBLAN", having extremely low levels of cationic and oxide impurities, and useful for example, in low-loss IR transmission applications.
The metal fluorides for IR glass preparation must have very low levels of impurity, with respect, for example, to transition elements, rare earths, and hydroxide ion in order to minimize absorption in the 2 to 4 micron range of the IR region of the spectrum. Furthermore, particulate materials such as metal oxides, carbides, carbon, coke, phosphides, and the like, must be essentially absent since they serve as scattering centers for the electromagnetic radiation and thus produce undesirable attenuation. Such impurities might seem to be out of place in purified fluoride compounds which typically are used in glass manufacture, however, in the minute quantities with which the present invention is concerned, these impurities can easily be introduced by the use of particular materials present in chemical process equipment and even from available purified reagents and metals. In this regard, the variety of cationic materials and metal complexes and compounds present in the less than trace amounts involved herein, and which must be rendered involatile or removed producing the ultra pure product salt, described herein is quite large. The problems presented by the presence of such materials have been the subject, for example, of "Proceedings for the 4th International Symposium on Halide Glasses", 1987.
The rigorous purity required for such glasses have previously often been beyond the capability of the available technology, being, e.g., at about the level of just a few parts of impurity per billion (ppb) or less of the desired salt. Commercially available fluorides, even those sold as very high purity, have such impurities at the 10 to 1000 parts per million (ppm) level. Thus, there s a recognized need for a greater improvement in purity than presently available, such as, e.g., a one thousand to one million-fold betterment.
The presently known purification techniques for the production of zirconium and hafnium fluorides are recrystallization, solvent extraction, and the anhydrous methods of electromotive series displacement (ESD)-augmented distillation of ZrF.sub.4 (the subject of U.S. Pat. No. 4,578,252), and chemical vapor purification (CVP) of zirconium metal (the subject of U.S. Pat. No. 4,652,438).
The recrystallization and solvent extraction methods accomplish purification at the ZrOCl.sub.2 stage, but there is substantial impurity pickup, e.g., 2 to 10-fold, in the succeeding steps to ZrF.sub.4. Furthermore, these methods tend, undesirably, to leave oxide and hydroxide impurities in the ZrF.sub.4.
The anhydrous methods, wherein purification capability is based essentially on the high reducing power of solid reducing agents or electrodes which reduce, e.g., metal chlorides, and effect a lowering of impurity volatilities, are restricted in practical application by the limitations of mass transport in the proposed schemes. For example, in the CVP method, the calculated iron level in the product ZrF.sub.4 is about 0.007 ppb, i.e., the equilibrium vapor pressure of FeCl.sub.2 in contact with Zr metal at 923.degree. K. In fact, however, the achieved level was about 200 ppb, a discrepancy of over 10.sup.4 as shown in Table 2 below.
In the known distillation purification of ZrF.sub.4 from BaF.sub.2 -ZrF.sub.4 solutions, there is a similar shortfall of purification. The distillation without ESD yields a product with unacceptable iron impurity levels, while the ESD-augmented option which gives further improvement, also falls far short of theoretical thermodynamic limits. Also, repeated distillation does not appear to yield further improvement. These data indicate a failure, due to limitations of mass transport, to achieve or approach the desired purity level of about 10 to about 100 ppb.
Objectives, therefore, of the present invention are to provide a method for the preparation of selected metal fluoride product salts and their precursor salts, of heretofore unattainable purity, wherein the method has highly acceptable requirements of energy input, chemical ingredient expense, and apparatus complexity. Further, it is an objective to provide an economical and relatively non-complex process, which, in operation, is relatively easy to control, i.e., temperature, volatilization rate, precursor salt take off, and subsequent product salt formation, is readily adaptable to batch, semi-continuous or continuous operation, and which can efficiently and effectively produce relatively large quantities of precursor salt and its fluoride product salt, e.g., several times the quantities per batch, heretofore possible by means of the aforementioned known processes.
These and other objects hereinafter appearing have been attained in accordance with the present invention which is defined in its broad sense as a process for purifying a chloro, bromo or iodo precursor salt of zirconium, hafnium or aluminum containing impurities, comprising providing a molten thermal body of one or more alkali or alkaline earth metal halides and said precursor salt, maintaining said body at a vaporizing temperature between about 150.degree. C. and about 460.degree. C. to vaporize said precursor salt away from its impurities while effecting a reducing condition in said body with a reducing agent which is fluid at said vaporizing temperature, is mobile in said body, and is non-reducing of said precursor salt metal, and isolating the volatilized pure precursor salt from said thermal body.